1. Field of Invention
The present invention relates generally to circuits for controlling diode switches by a ratio frequency input and, more specifically, to a circuit utilized as a microwave receiver converter having a hybrid structure.
2. Description of Prior Art
It is known that one of the most important problems to be faced in the design of receiver converters in that of minimizing as far as possible the noise of these devices.
However, it is evident that the solution of the problem lies in achieving such objectives as the obtaining of special electrical characteristics (e.g., the bandwidth and range of operation, linearity for high input signals, impedance presented at the working frequency, etc.) or structural and technological characteristics (MIC type structures or classical type structures designed with more or less sophisticated functions or operation, diodes in CHIP form or diodes incapsulated in robust and manageable packages) or functional characteristics (e.g., degree of deterioration of the characteristics due to failure of a diode in the case of a balanced converter; or due to variation of the power of the local oscillation (L.O.): capacity of overcoming special mechanical or electrical requirements, etc.).
In the literature (Torrey and Whitmar--"Crystal Rectifiers" MIT vol. 15; Mohr and Kwitt--"A Note on the Optimum Source Conductance of Crystal Mixers"--IRE MTT 8 pp 622; Barber--"Noise Figure and Conversion Loss of the Schottky Barrier Mixer Diode"--IEEE MTT 15 p. 629-635; Stracca--"Noise in Frequency Mixers Using Non-linear Resistors"--A. F. 1971 pp 484-505; Caroli--"Considerazioni sui Convertitori di Oscillazioni a Microonda"--Genoa Congress 1968; Kelly--"Fundamental Limits on Conversion Loss of Double Sideband Mixers". TRANS. IEEE MTT November 1977) typical and experimental analyses are well known which give the general criteria to be followed to operate in this way and which identify the main problem to be solved as being an efficient control of the current due to the image frequency and to various idle frequencies, and therefore the need to maintain high values for the signal-to-noise ratio.
That means that maximum care must be paid to ensuring that the non linear devices used in the converter are correctly terminated at the image and idle frequencies.
The more accurately these terminations are controlled by the network used to actuate the converter, the more easily characteristics favorable in the way described above and their repetivity can be achieved.
The problem of controlling idle frequencies has resulted in a preference for balanced configuration converters (originally used for their capability of cancelling the out-of-band noise of klystrons), since as is known from the literature, they allow the separation of some of the idle frequencies between the local oscillation (LO) and radio frequency (RF) ports and therefore simplify the problem of controlling them.
The problem of the control of the image frequency on the other hand is very complex and has been tackled in the present-day technique by the insertion of a suitable structure in the RF path which presents a high impedance at that frequency (e.g. a band-pass filter at the signal frequency, selective TR, a band-stop filter at the image frequency).
This solution to the problem is not without disadvantages, since, besides involving the complication of an added structure, it necessitates making adjustments on changing frequency.
In the field of balanced converters, those having a hybrid co-axial-waveguide structure are particularly convenient since this, as is well known in the current technique, is ideally suited to the use of diode converters which are realized in well built containers and are easily replaceable in case of failure.